Amyotrophic lateral sclerosis (ALS) is the 3rd most common human neurodegenerative disease with an adult onset. It is a fatal paralytic disease of motor neurons (MNs) without any effective treatments. Novel mechanism-based targets need to be identified for drug discovery relevant to diseased MNs. Some forms of ALS are inherited and are caused by mutations in the superoxide dismutase-1 (SOD1) gene, thus providing a clue about MN vulnerability. Many different SOD1 mutations occur, but the mechanisms of human SOD1 (hSOD1) toxicity to MNs are unresolved. Importantly, the autonomy of the MN degeneration in ALS is an important unresolved problem. We hypothesize that skeletal muscle is a primary site of pathogenesis in ALS that triggers MN degeneration. We have created new transgenic (tg) mice with skeletal muscle-specific expression of hSOD1 gene variants. These hSOD1mus tg mice develop neurologic and pathologic phenotypes consistent with ALS. Using these novel mice we propose to study skeletal muscle as a disease-causing entity in ALS. In Aim 1 we will analyze the age-related neurologic and pathologic phenotypes of hSOD1mus tg mice. We hypothesize that the mechanisms of MN degeneration in our hSOD1mus tg mice are consistent with distal axonopathy and target deprivation-induced apoptosis. In Aim 2, we will analyze the involvement of oxidative stress and activation of the mitochondrial permeability pore in skeletal muscle as mediators of muscle pathology in hSOD1mus tg mice. In Aim 3 we will use cultured cells to examine if hSOD1 expression in skeletal muscle cells alters their intracellular redox state, Ca2+ handling, and ion channel function and disrupts the neuromuscular junction, thus provoking MN degeneration. The work can lead to new concepts about the non-autonomous death of MNs in ALS pathogenesis. The discovery of instigating toxicities or disease progression determinants within skeletal muscle would be very valuable for development of new effective therapies in the treatment and cure of ALS.